1. Field of the Invention
The present invention relates, in general, to the field of an apparatus and method for accurately sawing a work piece into two or more sections. More particularly, the present invention relates to an endless wire saw and a method for making an endless wire saw for cropping and/or slicing crystalline ingots, such as relatively large diameter polysilicon and single crystal silicon ingots.
2. Description of the Related Art
The vast majority of current semiconductor and integrated circuit devices are fabricated on a silicon substrate. The substrate itself is initially created utilizing raw polycrystalline silicon having randomly oriented crystallites. However, in this state, the silicon does not exhibit the requisite electrical characteristics necessary for semiconductor device fabrication. By heating high purity polycrystalline silicon at temperatures of about 1400 degrees, a single crystal silicon seed may then be added to the melt and a single crystalline ingot pulled having the same orientation of the seed. Initially, such silicon ingots had relatively small diameters of on the order of from one to four inches, although current technology can produce ingots of 150 mm (six inches) or 200 mm (eight inches) in diameter. Recent improvements to crystal growing technology now allow ingots of 300 mm (twelve inches) or 400 mm (sixteen inches) in diameter to be produced.
Once the ingot has been produced, it must be cropped (i.e. the "head" and "tail" portions of the ingot must be removed) and then sliced into individual wafers for subsequent processing into a number of die for discrete or integrated circuit semiconductor devices. One method for cropping the ingot is through the use of a band saw having a relatively thin flexible blade. However, the large amount of flutter inherent in the band saw blade results in a very large "kerf" loss and cutting blade serration marks which must then be lapped off.
One technique of slicing a semiconductor ingot into wafers is the slurry saw. The slurry saw comprises a series of mandrels about which a very long wire is looped and then driven through the ingot as a silicon carbide or boron carbide slurry is dripped onto the wire. Wire breakage is a significant problem and the saw down time can be significant when the wire must be replaced. Further, as ingot diameters increase to 300 mm to 400 mm the drag of the wire through the ingot reaches the point where breakage is increasingly more likely unless the wire gauge is increased resulting in greater "kerf" loss. Importantly, a slurry saw can take many hours to cut through a large diameter ingot.
A much preferred technique for slicing an ingot into wafers is disclosed in copending patent application Ser. No. 08/888,952, filed Jul. 7, 1997 and entitled Apparatus and Method For Slicing A Workpiece Utilizing A Diamond Impregnated Wire, the disclosure of which is hereby incorporated by reference in its entirety. This technique is a method for slicing a work piece, in particular, a polysilicon or single crystal silicon ingot utilizing a length of diamond impregnated wire in which the work piece (or ingot) is rotated about its longitudinal axis as the diamond wire is driven back and forth orthogonally to the longitudinal axis of the work piece and advanced from a position adjoining the outer diameter ("OD") of the ingot towards its inner diameter ("ID"). In this manner, the diamond wire cuts through the work piece at a point substantially tangential to the circumference of the cut instead of through up to the entire diameter of the piece. Through use of this technique, polysilicon or single crystal silicon ingots of 300 mm to 400 mm or more may be sliced into wafers relatively quickly, with minimal `kerf" loss and less extensive follow-on lapping operations.
There is no known continuous wire saw loop that can be used to make these extremely fine cuts. Consequently, the apparatus for sectioning a substantially cylindrical crystalline work piece with this technique uses a relatively long length of wire having a plurality of cutting elements affixed thereto which has both ends wrapped around a capstan which alternatingly rotates first in one direction and then an opposite direction, while the work piece rotates continuously in one direction or alternately in opposite directions to the movement of the wire. This technique and apparatus results in faster, finer cuts than the slurry saw can produce.
Laser Technology West, Limited, Colorado Springs, Colo., a manufacturer and distributor of diamond impregnated cutting wires and wire saws, has previously developed and manufactured a proprietary diamond impregnated wire marketed under the trademarks Superwire.TM. and Superlok.TM.. These wires comprise a very high tensile strength steel core with an electrolytically deposited surrounding copper sheath into which very small diamonds (on the order of between 20 to 120 microns) are uniformly embedded. A nickel overstrike in the Superlok wire serves to further retain the cutting diamonds in the copper sheath.
The band saw technique discussed first above requires an endless loop saw blade band. Such an endless loop would be extremely efficient at cutting ingots. In addition, multi-loop band saws machines could be constructed to make simultaneous cuts and thus greatly shorten the processing time for these ingots. However, in order to accurately cut without significant kerf losses and scoring of the cut surfaces, an extremely fine wire saw loop would be required instead of a band, using a wire such as the diamond impregnated wire described in the previous paragraph.
Unfortunately, known attempts to make a suitable wire saw loop that can withstand the stresses of operation have all failed. These wire saw wires are extremely small diameter wires, on the order of 0.005-0.015 inch diameter wire. The formation of a wire loop requires welding the ends of the wire together. The welding of such wire materials together forms a brittle region at the weld, thus predisposing the wire loop to failure at the weld location. Conventional teachings require that the ends of the wire must be either shaped to have a slanted end surface so that the ends overlap in order to have a sufficient surface area at the weld location or shaped to provide blunted and rounded tips that are abutted and then melted together during the weld process. The overlapping and blunting results in a thickened, embrittled region at the weld which can bind in the saw kerf, leading to immediate wire breakage. Conventional annealing and heat treating of the weld leads to weakened wire strengths at the weld, again leading to premature failures. Thus an appropriate closed wire saw loop that can be used is lacking in the prior art. Attempts to fabricate a suitable wire loop in the past have all resulted in unacceptable breakage at the weld location. Consequently, closed wire saw loops are simply unknown in the semiconductor wafer manufacturing industry.
Therefore there is a need for diamond impregnated closed wire saw loop for use in cutting a work piece such as a semiconductor crystal ingot into thin, accurately cut wafers, an apparatus for such a saw loop, and a method of making such a wire saw loop that overcomes the above problems.